Conventional xDSL services, including ADSL and VDSL, are deployed over copper loops that can have a wide range of topologies and characteristics. As such, these loops can have many undesirable characteristics that will negatively impact the performance of either ADSL or VDSL modems. One such loop characteristic that can negatively affect the performance of an xDSL modem is a bridge tap. A bridge tap is a relatively short segment of twisted pair wire that is “bridged” onto the main loop at some distance, d, from the customer's modem. A bridge tap can, and in many cases does, exist within a customer's home itself.
Single-ended line tests (SELT) are conventionally used in xDSL systems to identify loop impairments such as bridge taps. As is known in the art, SELT involves transmitting a known signal, capturing and measuring the resultant S11 frequency domain data, and then performing an inverse Fourier transform on the S11 to derive the time domain equivalent of the frequency domain S11 data. Such a time domain signal will typically exhibit peaks at the points of discontinuity in the loop. So, by analyzing the peak information one can determine various characteristics of the loop. See, for example, U.S. patent application Ser. Nos. 14/341,538 and 14/341,576, commonly owned by the present assignee, the contents of which are incorporated by reference herein in their entirety.
Nevertheless, certain loop characteristics cannot be accurately determined merely by analyzing peaks in the time domain S11 signal. Moreover, conventional techniques do not adequately address how to dynamically mitigate certain characteristics which can adversely affect xDSL transceiver performance. Accordingly, a need remains to more accurately determine loop characteristics and to mitigate the effects of characteristics that can negatively affect xDSL modem performance.